In recent years the mounting density of electronic components has continued to rise, and very fine interconnect substrates are required. There is a limit to the degree to which the pattern of an interconnect substrate can be made finer when forming the interconnect pattern on one surface of the substrate only, and therefore built-up interconnect substrates have sometimes been used.
However, a built-up interconnect substrate requires repeated steps of applying an interconnect and an insulating layer, and photosensitive resin and other expensive materials, and it has therefore not been possible to reduce the cost to that of a conventional printed interconnect substrate.